It is known that the performance of settlers depends in a large measure on their internal hydraulics: the nature of the flow, short-circuits, recirculations, retention time. Some domain specialists evaluate the drop in efficiency due to poor internal hydraulics of the settler at more than 50%.
It is known moreover that this internal hydraulics itself depends on a plurality of factors and in particular on the following factors:
the geometry of the settler; PA1 the device for distributing the liquid at the entrance to the settler; PA1 the device for recovering the treated liquid; PA1 the device for recovering the settled matter and for recycling it; PA1 the relative arrangement of the three devices specified above. PA1 1) regular distribution of the incoming flow in the two directions, vertical and horizontal, of the stream cross-section of the settler; PA1 2) optimal dissipation of the input energy; PA1 3) reduction in the impact of the secondary currents: density currents, thermal currents; PA1 4) reduction in the impact of the throughput variations; PA1 5) participate in the flocculation of the matter in suspension; and PA1 6) minimize the disturbances of the settled matter lying on the bottom of the settler ("sludge bed").
Among the various factors listed above, the distributing device, by virtue of its position at the origin of the chain, is the one which most affects the hydraulic behavior of the settler.
It is known that in order to yield good results, a liquid distributing device at the entrance of a settler must accomplish the following functions:
It is known that fluid jets entering a relatively "stationary" fluid of like nature exhibit the property of transferring some of their momentum to it. This property is manifested through entrainment of a certain mass of the ambient fluid. The jet is then progressively slowed and dispersed at the same time as its field of action broadens, promoted by the lateral extent of the fluid region.
In practice, all devices for feeding reactors and especially settlers are manifested through the formation. of jets. Now, the feed devices are always very small in relation to the dimensions of the reactors. The non-infinite dimension of the reactors prevents the feed jets from spreading in the manner of free jets by virtue of the existence in particular of edge effects due to the walls, of the free surface, etc. The result is that the jets are deflected at the whim of the geometries of the reactors and they thus become the direct cause of poor internal hydraulics.
It is indeed known that the existence of a wall has the effect of limiting the lateral field of the jet. The fluid which is entrained upstream can only be replaced by reverse currents (backflows) originating from downstream and causing the deflection of the jet.
The present proprietor has devoted himself to various numerical simulations making it possible to demonstrate such a phenomenon which has moreover already been observed in certain industrial installations constructed previously.
In FIGS. 1 to 6 of the appended drawings are represented various flow configurations obtained during simulations carried out by the proprietor, of a number of known devices.
FIG. 1 refers to a free-jet feed system. This case does not exist in practice but its study makes it possible to illustrate the fundamental properties of jets (especially entrainment of the ambient fluid, slowing and expansion of the jet, etc.).
FIGS. 2 and 3 refer to a feed system including an inlet via a jet which is more or less centered between two walls. FIG. 2 shows the configuration of the flow obtained with the aid of an off-centred jet. Its deflection is seen to be appreciable. Since a perfectly centred jet cannot be produced in practice, jets having a very small eccentricity (of the order of a few %) have been simulated. The result illustrated by FIG. 3 is at the very least surprising: a very small eccentricity is sufficient for the jet to be deflected fully (toggle system, high instability).
FIG. 4 refers to a jet feed system (spillway) furnished with a siphon unit. The use of such a siphon unit is widespread since, apart from its obvious action of deflecting the entrance jet, it is ascribed supposed advantages such as, in particular, the dissipating of the input energy or the improving of the velocity distribution. The present proprietor has carried out a particularly thorough study of such a device. The conclusion is that in all cases the results obtained are similar to those of more or less off-centred jets.
In all cases, the result is the same: only a small fraction of the cross-section presented to the flow of the fluid is used. The incoming fluid is concentrated there in the form of a deflected jet. The rest of the cross-section is occupied by an induced return current.